Fix compile warnings with glib newer than 2.15
Only use _FORTIFY_SOURCE when building for Release

In file included from /usr/include/string.h:25:0,
                 from /usr/include/qt4/QtCore/qbytearray.h:48,
                 from /usr/include/qt4/QtCore/qstring.h:46,
                 from /usr/include/qt4/QtCore/qobject.h:48,
                 from /usr/include/qt4/QtCore/QObject:1,
                 from /home/jonas/Dokumente/OpenSource/KeePassX/src/autotype/AutoType.h:21,
                 from /home/jonas/Dokumente/OpenSource/KeePassX/src/autotype/AutoType.cpp:18:
/usr/include/features.h:327:4: Warnung: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Wcpp]
 #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)